Cloudy with a chance of Physics

I don’t know. That’s a bit of a desperate title. But in exchange, a rather nice cloud formation, don’t you think?

This was from the sky over Los Angeles yesterday evening (a shot of the sky in the other direction is to the right – click for larger), and my first thought was “what’s the physics behind these beautiful structures?” There’s enough regularity here to expect there to be a mechanism, but I do not know what it is. Some combination of atmospheric conditions like wind speed, temperature, perhaps some layering of different bodies of air, and so forth, resulted in this and I’d love to know more. What factors set the roughly regular size of the structures, their pretty uniform distance apart, etc? (These are typical physicist’s questions, in case you’re wondering.) I found an article on Wired with lots of nice pictures and the beginnings of some discussions of the effects (be sure to visit all seven pages to see the different spectacular cloud types discussed), but all it did was whet my appetite. There’s not much there and it certainly did not say anything about this sort of formation. But it is a start. Apparently it is partly because there’s not a large community out there studying cloud formations for their own sake. Seems to me they ought to team up with the sand-pile people. This stuff could turn out to be useful for something, I’ll bet…

I just finished a great book about the Romantic Age of Science, “The Age of Wonder: The Romantic Generation and the Discovery of the Beauty and Terror of Science” by Richard Holmes

One of many fascinating people profiled was a British chemist, manufacturer and amateur meteorologist, who invnted the cloud classification system in the early 1800s–one cool thing was that his work led to correspondence with Goethe and inspired poems by Goethe and Shelley, one example of how the arts, humanities and science we not as disconnected in the 1700s and 1800s.

These are internal gravity waves. The water vapor acts as a flow visualizer. Where the air rise it cools and water condenses to from clouds. Where the air sinks, it warms and the water vapor evaporates.

Internal gravity waves are similar to the waves you see on the surface of the ocean. There, there is a sharp discontinuity in the density going from water to air. In the atmosphere, the density varies smoothly but still supports waves.

The Wikipedia entry on internal gravity waves is not bad, it gives the equations of motion and the dispersion relation. More detail can be found in an atmospheric dynamics textbook.

“so I’m open to being convinced about the best way to think about this…”

The best way to think about any wave is in terms of the restoring force. Imagine a stratified atmosphere, less dense fluid above denser fluid. Now imagine taking a tiny parcel of air and lifting it up and letting go. Initially the parcel is denser than the surrounding air so it sinks. When it reaches neutral buoyancy it is moving and so continues sinking past this point. It is now denser than the surrounding air so it slows, and eventually starts rising.

Just like in a pendulum, gravitational potential energy is exchanged with kinetic energy, but the gravitational potential energy is due to buoyancy in a stratified fluid.